1. Field of the Invention
The present invention is directed to an apparatus and method for determining the presence of specific elements or compounds within an object. The present invention is more specifically directed to an apparatus and method that utilizes a measurable change in the radiation absorption and/or transmission characteristics of an object due to resonant frequency coupling between nuclei of certain elements within the object to identify the presence of the specific elements or compounds within the object.
2. Description of the Prior Art
One of the most important applications of methods and devices designed to non-intrusively detect the presence of specific elements and compounds is the provision of improved security for passengers in domestic and international travel. There is a significant need in both the governmental and private sector for a rapid and reliable technique for the detection of explosive devices within luggage or parcels carried onto air craft or brought into buildings. The demand for a fast and effective device for scanning such objects to determine the presence of explosives has only increased in recent years.
Explosive devices hidden in packages, letters, luggage, and elsewhere are easy to detect with metal detectors if they incorporate substantial quantities of metal. There are many different techniques that will locate such metallic explosives by detecting the presence and quantity of a number of types of metals. However, it is becoming increasingly more likely that such hidden explosives will be made from non-metallic materials. It is well known that many of these non-metallic explosives may be detected by sensing the amount of nitrogen present in the object being analyzed.
One technique of detecting the presence of nitrogen involves subjecting the object under consideration to a thermal neutron flux environment. Neutron capture can be measured by a resulting emission of gamma rays from the nuclei of the nitrogen atoms if they are present. The gamma ray spectrum that results from such neutron capture can, to some extent, be associated with certain characteristics of particular nitrogen containing compounds.
U.S. Pat. No. 3,124,679, issued to Tittman, et al utilizes the neutron capture effect to determine the presence of nitrogen in objects. The material being examined is irradiated with neutrons, and the resultant gamma rays are measured at appropriate angles from the impinging radiation. Certain gamma rays that have energy characteristics typical of the elements that are desired to be detected are selected and measured. Unfortunately, the technique described in the Tittman patent examines only the total nitrogen content and is, therefore, not capable of distinguishing explosives from every day articles that contain like amounts of nitrogen such as wool, silk, leather, and certain food products.
U.S. Pat. No. 3,832,545, issued to Bartko also applies the neutron capture principle to detect the presence of nitrogen within baggage. Bartko utilizes an array of gamma ray detectors to provide a silhouette or profile of the object under examination. Unfortunately, this silhouette is without fine definition and the possibility of mistaking an innocent nitrogen containing object within the luggage for an explosive device, or vice versa, still exists. In addition, the Bartko device requires an optimal thermal flux density within the radiation chamber that is not readily achievable.
U.S. Pat. No. 4,851,687, issued to Ettinger, et al also utilizes neutron capture to determine the presence of nitrogen in objects and improves upon the Bartko patent by incorporating an array of thermal neutron sensors within the examination chamber to monitor the amount of thermal neutrons and adjust the flux within the chamber to maintain flux density at an optimum level. The object is scanned rather than merely illuminated in this manner and adjustments are continuously made so as to provide a distribution profile of the nitrogen contained within the object. This more accurate profile produced by the Ettinger design does improve upon previous efforts at applying the thermal neutron principle, but still leaves a significant possibility for errors and false alarms.
A second approach to detecting the presence of nitrogen within objects under consideration involves the application of nuclear magnetic resonance or NMR. Nuclear magnetic resonance is defined as a resonance achieved, whereby energy is transferred between a magnetic field and nuclei placed within the magnetic field, that is of a strength sufficient to at least partially decouple the nuclei from their orbital electrons. The relationship between the frequency at which maximum energy is absorbed by the atomic nuclei of the element, the resonant frequency, and the magnetic field intensity is indicative of the particular element involved.
The NMR detection technique, in general, is old and is found in a number of previously described detection systems. One of the difficulties with these various systems using NMR techniques involves the fact that significant quantities of the material must be present to concentrate the elements of interest so that a noticeable response is achieved. The signals obtained by NMR techniques are typically very small and require high resolution detection equipment.
U.S. Pat. No. 4,166,972, issued to King, et al describes a method and apparatus for enhanced nuclear magnetic resonance discrimination and detection that focuses on the response of combinations of first and second distinct atomic elements that are placed within a variable magnetic field. Variations in the magnetic field alter the NMR frequency of the first element of interest (hydrogen for example) so as to coincide with a nuclear quadrupolar resonance (NQR) of a second element of interest (nitrogen for example). Energy is transferred in an enhanced fashion between the nuclei of the first element and the nuclei of the second element, and thereby reduces the NMR response time of the first element, which improves the detectability of that element. It is the detectability provided by the '972 patent that improves over the previous applications of the NMR technique. The '972 patent nonetheless is designed only to detect characteristic NMR changes associated with elements that are still quite prevalent in non-explosive, non-suspect compounds. The possibility, therefore, for characterizing an innocent object as an explosive still exists.
U.S. Pat. No. 4,296,378, also issued to King, describes a method of application similar, in some respects, to that of the '972 patent, but does not distinguish between substances in which the desirable cross relaxation occurs, and those substances that merely contain nuclei of the first kind of element with similar NMR relaxation times.
U.S. Pat. No. 4,514,691, issued to De Los Santos, et al likewise focuses on the NMR effects upon a first elemental nuclei when subjected to a magnetic field. (Typically this first element nuclei is hydrogen). The De Los Santos patent then measures the NMR response of elemental hydrogen in compounds by sending out an interrogation pulse from a second magnet and receiving echoes back that are stored and analyzed. The NMR response of certain types of explosives which contain elemental hydrogen is compared to the signals received and used to identify the presence of such explosives. While further limiting the range of elements that might be detected by the method, the De Los Santos patent still, by focusing on the hydrogen component in NMR technology, is faced with a range of error possibilities.
U.S. Pat. No. 4,887,034, issued to Smith discloses a method and apparatus for the detection of compounds containing both NMR exhibiting nuclei and NQR exhibiting nuclei. The Smith patent describes the use of both magnetic field changes and the application of pulse sequences in RF fields in various combinations. Variations in the magnitude of the NMR signals obtained as a result are used to characterize certain explosive substances. The Smith patent still, however, focuses on making measurements of the first, NMR type, element (typically hydrogen) rather than identifying compounds associated with characteristics of the second, NQR type, element.
As described above in many of the referenced patents, explosives detection is typically based upon determining the presence of several different elements together. The elements, as can be detected, come in different ratios for different explosives and can, therefore, be identified with different signatures. Explosive compounds typically contain the elements of hydrogen, nitrogen, carbon, and oxygen. The relative amounts of each of these elements varies and, in some explosives, some of these elements may not be present. The response of elemental hydrogen to NMR techniques, as described above, is high compared to the response of nitrogen. This is why, in most cases, the hydrogen nuclei is considered the first element with varying NMR frequencies that coincides with variations in magnetic field strength, and nitrogen is the second element with a fixed NQR frequency. Unfortunately, hydrogen is typically a constituent of materials near or surrounding the object being examined. All of the above referenced inventions, therefore, that apply the use of NMR technology to the examination of an object fail to sufficiently limit the scope of the objects detected because of their focus on the hydrogen components.
In a similar manner, those techniques described above that utilize thermal neutron capture as the examining radiation only identify the presence of nitrogen rather than its presence in a particular compound or with some other element typical of an explosive. It would be advantageous, therefore, to employ an apparatus and method that focussed not on the hydrogen component of an explosive compound, but rather the nitrogen component, and would thereby narrow the field of possible identifications of the compound, and in addition employ the "fingerprinting" capabilities of the NMR and NQR techniques to further limit and distinguish the nitrogen compounds.